1. Field
One embodiment of the present invention generally relates to a disk drive including a servo control system and, more particularly, to a servo control system obtained by improving an off-track position detecting method.
2. Description of the Related Art
In a conventional disk drive typified by a hard disk drive, a servo control system which controls a head to position the head at a target track (target cylinder) position by using servo information recorded on a disk medium is incorporated.
The servo control system uses servo information read by a read head to calculate a track address to identify a target track and an off-track position (PES) which is a positional error from a track center of the target track. The servo control system uses a servo burst pattern included in servo information to calculate an off-track position.
In this case, as a scheme which detects an off-track position of a read head, in addition to a burst detecting scheme which uses a servo burst pattern, a phase difference detecting scheme is known (for example, see Jpn. Pat. Appln. KOKAI Publication No. 6-231552).
In this phase difference detecting scheme, schematically, a discrete Fourier transform (DFT) process is performed to a servo reproduction signal read by a read head and reproduced by a signal processing circuit (read channel) to generate a sine component value and a cosine component value. Furthermore, the ratio (tangent) of the sine to the cosine is subjected to arctangent transform to detect an off-track position corresponding to a phase difference from the track center.
In a conventional phase difference detecting scheme, a servo pattern different from a servo burst pattern of the burst detecting scheme is generally used. On the other hand, of burst values A, B, C, and D obtained from the servo burst pattern, a servo operation “A-B” is regarded as an almost sinusoidal component value, and servo operation “C-D” is regarded as an almost cosinusoidal component value, so that an almost correct off-track position can be detected by an arctangent transform process performed by an inverse function approximate expression. More specifically, the phase difference detecting scheme using a servo burst pattern can be applied.
However, in an actual disk drive, since off-track characteristics based on a burst pattern include a high-order distortion (or an asymmetrical distortion) or an even function, an error over a tolerance probably occurs between an off-track position calculated by an arctangent transform process using a simple inverse function approximate expression and a genuine off-track position. Such deterioration in detection accuracy of an off-track position (positional error of a head with respect to a target position) causes deterioration in servo performance of a disk drive.